In this practical we will write a simple function that simulates rolling one or more dice.

The function sample()

The function sample allows us to draw values from a given set of values, with or without replacement.

It has arguments

  • x: a vector of values to draw from
  • size: the number of values to draw
  • replace: can we choose the same element of x multiple times?
  • prob: a vector of probability weights (by default all elements of x have equal probability to be drawn)

One die

Task 1

Write the syntax to simulate a single role of a die using the function sample() (no function yet).

You need to specify the arguments x and size.

Solution 1

sample(1:6, size = 1)
## [1] 3

Task 2

Write your own function that simulates this one role of a die.

The general form of a function is
<function name> <- function ( <arguments> ) {
  <function body>
}

Solution 2

We use the exact syntax from the previous solution as our function body. Since our function does not need any input (it will always do the same thing) the function does not need any arguments.

die <- function() {
  sample(1:6, size = 1)
}

die()
## [1] 3
die()
## [1] 5
die()
## [1] 4

Multiple dice

Task 1

Take another look at the description of the function sample(). How would you need to change the syntax from Task 1 from above to simulate rolling two dice?

You need to change what is passed to the argument size and you need to specify the argument replace.

Solution 1

sample(1:6, size = 2, replace = TRUE)
## [1] 1 5

Task 2

  • Write another function that simulates rolling multiple dice.
  • Write the function so that the number of dice is flexible.
This new function needs to have an argument, which then is passed to sample().

Solution 2

dice <- function(n) {
  sample(1:6, size = n, replace = TRUE)
}

dice(2)
## [1] 6 3
dice(1)
## [1] 2
dice(10)
##  [1] 4 3 1 2 6 4 2 2 3 5

Task 3

  • What happens when you use the function dice() without specifying the argument n?
  • Do you know or can you guess why that happens?

Solution 3

dice()
## [1] 3 4 1 3 3 6

By default, the function dice() will role 6 dice.

This is due to the behaviour of the function sample(). In the help file for sample() (run ?sample) we can read in the section Details:

For sample the default for size is the number of items inferred from the first argument, so that sample(x) generates a random permutation of the elements of x (or 1:x).

What can we learn from this?

  • Read the help files!
  • Think about / try what will happen if “wrong” input is given to your function!

Task 4

Re-write the function dice() so that by default only one die is used.

Solution 4

dice <- function(n = 1) {
  sample(1:6, size = n, replace = TRUE)
}

dice()
## [1] 1
dice()
## [1] 4
 

© Nicole Erler